This invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to simultaneously provide stabilization and continued flexibility and proper anatomical motion, and more specifically to such a device which utilizes a wave washer as a force restoring element.
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes which can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art which achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Referring now to FIGS. 1 and 2, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages 10 generally comprise tubular metal body 12 having an external surface threading 14. They are inserted transverse to the axis of the spine 16, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in FIG. 2 the pair of cages 10 are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages 10 are generally inserted side by side with the external threading 14 tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) below. The cages 10 include holes 18 through which the adjacent bones are to grow. Additional material, for example bone graft materials, may be inserted into the hollow interior 20 of the cage 10 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage 10.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient""s spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which nearly completely mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.
It is, therefore, an object of the present invention to provide a new and novel intervertebral spacer which stabilizes the spine without promoting a bone fusion across the intervertebral space.
It is further an object of the present invention to provide an implant device which stabilizes the spine while still permitting normal motion.
It is further an object of the present invention to provide a device for implantation into the intervertebral space which does not promote the abnormal distribution of biomechanical stresses on the patient""s spine.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.
The preceding objects of the invention are achieved by the present invention which is a flexible intervertebral spacer device comprising a pair of spaced apart base plates, arranged in a substantially parallel planar alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) and coupled to one another by means of a spring mechanism. In particular, this spring mechanism provides a strong restoring force when a compressive load is applied to the plates, and may also permit limited rotation of the two plates relative to one another. While there are a wide variety of embodiments contemplated, two embodiment families are described herein as representative of preferred types. Each of these embodiments includes a wave washer utilized as its force restoring element.
More particularly, with respect to the base plates, which are similar in all embodiments, as the assembly is to be positioned between the facing surfaces of adjacent vertebral bodies, and as such need to have substantially flat external surfaces which seat against the opposing bone surfaces. Inasmuch as these bone surfaces are often concave, it is anticipated that the opposing plates may be convex in accordance with the average topology of the spinal anatomy. In addition, the plates are to mate with the bone surfaces in such a way as to not rotate relative thereto. (The plates rotate relative to one another, but not with respect to the bone surfaces to which they are each in contact with.) In order to prevent rotation of a plate relative to the bone, the upper and lower plates may each include outwardly directed spikes which penetrate the bone surface and mechanically hold the plates in place. Alternatively, the base plates may be coupleable to other securing means for holding the present invention in place.
It is further anticipated that the plates may include a porous coating into which the bone of the vertebral body can grow. (Note that this limited fusion of the bone to the base plate does not extend across the intervertebral space.)
Between the base plates, on the exterior of the device, there is included a circumferential wall which is resilient and which simply prevents vessels and tissues from growing into the interior of the device. This resilient wall may comprise a porous fabric or a semi-impermiable elastomeric material, and serves a similar purpose to the naturally occurring annulus material which surrounds the cartilage of the intervertebral disc, which the present invention is designed to replace when conditions warrant. Suitable tissue compatible materials meeting the simple mechanical requirements of flexibility and durability are prevalent in a number of medical fields including cardiovascular medicine, wherein such materials are utilized for venous and arterial wall repair, or for use with artificial valve replacements. Alternatively, suitable plastic materials are utilized in the surgical repair of gross damage to muscles and organs. Still further materials which could be utilized herein may be found in the field of orthopaedic in conjunction with ligament and tendon repair. It is anticipated that future developments in this area will produce materials which are compatible for use with this invention, the breadth of which shall not be limited by the choice of such a material.
As introduced above, the internal structure of the present invention comprises a force restoring member, which provides a restoring force when compressed. More particularly, it is desirable that the restoring forces be directed outward against the opposing plates, when a compressive load is applied to the plates.
In addition, in certain select embodiments, it is necessary that the restoring force providing subassembly not substantially interfere with the rotation of the opposing plates relative to one another. In the first embodiment, the force restoring subassembly does not interfere with the rotation of the plates. In the second preferred embodiment, the spring subassembly substantially inhibits any rotation of the plates relative to one another. As further mentioned above, the force restoring member comprises at least one wave washer.
More particularly with respect to the first embodiment, in which the restoring force providing subassembly does not interfere with the rotation of the plates, the restoring force providing subassembly comprises a single wave washers. Wave washers resemble simple round washers which comprise a flat round ring, except that while maintaining a constant curvature of radius in the plane normally defined by the washer, the washer rises and falls in a wave-like around its circumference. Stated alternatively, a standard washer is a relatively planar ring-shaped object, confined to the x-y plane. The wave washer is ring-shaped object which undulates in the z-axis around its edge. The restoring force of a wave washer is proportional to the elastic properties of the material. In addition, the magnitude of the compressive load support and the restoring force provided by the wave washer may be modified by altering the thickness of the material in its radial extent, or in its z-axis, or, alternatively, by providing a radial slot in it. (For the purposes of this description, the top and the bottom of a wave washer shall be defined as the planes defined by the highest and lowest points of the circumferential undulations, respectively.)
As a compressive load is applied by a pair of plates against the top and bottom of a wave washer, the forces are directed against the arches of the undulating wave, and are also translated into a hoop stress which tends to radially expand the washer in its x-y plane. This force of deflection against the arches, and the hoop stress in the radial direction, are counterbalanced by the material strength of the washer. The strain of the material causes a deflection in the height of the washer and a slight radial expansion. (It shall be understood that in the embodiments in which the washer includes a radial slot, and in which the washer is constrained against radial expansion, for example by guard walls of the corresponding base plates, the deflection of the arches is not translated into a hoop stress, but rather into a circumferential deflection and closure of the slot.) Stated equivalently, a wave washer responds to a compressive load by deflecting compressively in z-axis, and either radially or circumferentially. The restoring force which is therefore proportional to the elastic modulus. With a slot formed in the washer, it expands and restores itself far more elastically than a continuous ring washer.
In general, the wave washer is one of the strongest configurations for a spring, and is highly suitable for use as a restoring force providing subassembly for use in an intervertebral spacer element which must endure considerable cyclical loading in an active human adult. In the first embodiment of the present invention, the ring-shaped wave washer is secured by a series of pins which extend upwardly and outwardly from one of the inner surfaces of one of the base plates. A compressive load applied to the base plates causes the corresponding compression of the washer, which in turn causes a restoring force to be applied to the base plates. The washer deflects appropriately under the load, only to spring back into its undeflected shape upon the unloading of the spring.
It shall be understood that the securing pins, as well as the exterior circumferential elastomeric material place constraints on the extent to which one base plate may rotate relative to the other. In this way the base plates may rotate relative to one another while maintaining a constant resilient capacity relative to the adjacent bone.
In a second embodiment of the present invention, a single modified wave washer, which is spiral-shaped, as opposed to being ring-shaped. More particularly, instead of the wave washer described in conjunction with the first embodiment, which defines a circumference of 360 degrees (in the solid variant) or less (the slotted variant), the wave washer of the second embodiment sweeps through an angle of 720 degrees or more (depending upon to specific anatomical needs). The undulations of the washer on the z-axis may be such that the arches are aligned, or misaligned. In the variant in which the arches are aligned, securing pins of the type introduced above (which extend upward and radially outward from the interior surface of one of the base plates) may be employed to hold the spiral wave washer in place. The alternative variant may be secured by a simple screw which locks one of the ends of the washer to the base plate.
Finally, inasmuch as the human body has a tendency to produce fibrous tissues in perceived voids, such as may be found within the interior of the present invention, and such fibrous tissues may interfere with the stable and/or predicted functioning of the device, preferred embodiments of the present invention may be filled with a highly resilient elastomeric material. The material itself should be highly biologically inert, and should not substantially interfere with the restoring forces provided by the spring-like mechanisms therein. Suitable materials may include hydrophilic monomers such as are used in contact lenses. Alternative materials include silicone jellies and synthetic collagens such as have been used in cosmetic applications. As with the exterior circumferential wall, which was described above as having a variety of suitable alternative materials, it is anticipated that future research will produce alternatives to the materials described herein, and that the future existence of such materials which may be used in conjunction with the present invention shall not limit the breadth thereof.